Can we have a progress bar when model()ing

[earowang]

Since fablelite depends on R6, we can have a simple progress bar like what dplyr::do() does https://github.com/tidyverse/dplyr/blob/bbcfe99e29fe737d456b0d7adc33d3c445a32d9d/R/progress.R for model().

We don't have to have an extra argument in model(), but provide an option getOption(fable.progress_bar, TRUE) to enable/disable it.

[Fuco1]

The same would be nice to have for forecast.

[mitchelloharawild]

A progress bar for forecast() and other methods will be added, some discussion for this can be found here: tidyverts/fabletools#268